The usual well-known ram-type blowout preventer now in general use, examples of which are shown in U.S. Pat. Nos. 2,883,141, 3,272,222 and 3,434,729, can effectively seal around only one size of pipe or other object extending through the preventer. As a result, it is normal practice to keep several sets of rams on hand and change the ram size each time the pipe size is changed; this is inconvenient as well as time consuming. If the blowout preventer stack is relatively inaccessible, as in offshore drilling, it becomes necessary to include several blowout preventers in the stack so that the several different pipe sizes used in the drilling of a well may be properly sealed. Such requirement greatly increases the cost of the equipment.
Some attempts have been made to develop ram-type preventers which are capable of sealing around pipes with a range of sizes and such preventers are disclosed in U.S. Pat. Nos. 2,194,258, 2,746,710, 2,760,751, 2,855,172, 2,947,508, 3,128,077, 3,897,038, 3,915,424, 3,915,425 and 3,915,426. For one reason or another, none of these preventers have fully satisfied the need for a ram-type variable blowout preventer. For example, the structure of U.S. Pat. No. 2,194,258 included plates which are subject to being bent by the higher well pressures; U.S. Pat. No. 2,746,710 provided relatively thick cantilever beams molded into the packing material and required the application of excessive pressure to move the packing into sealing position, thereby resulting in undue wear of the packing material; in U.S. Pat. No. 2,760,751, the swinging plates provide insufficient support to resist the pressures encountered in well drilling; U.S. Pat. No. 2,855,172 utilizes parallel separate rams but this imposes a severe strain on the packing material at the point where the pipe rubs during working; although showing an equalizer consisting of small rigid balls, U.S. Pat. No. 2,947,508 has disadvantages similar to U.S. Pat. No. 2,855,172; U.S. Pat. No. 3,128,077 has very low pressure capability, as well as an uneven distribution of force application to the sealing sleeve; U.S. Pat. Nos. 3,897,038, 3,915,424, 3,915,425 and 3,915,426 are objectionable because they provide complex anti-extrusion members which are difficult and expensive to manufacture. A serious shortcoming, which the present invention is directed to overcoming, of previous ram-type variable bore preventers, is the distortion of the circular relaxed shape of the sealing member to a more oval shape as the sealing members are urged toward and against the well member extending therethrough.
Aside from the ram-type, annular blowout preventers have been developed which are capable of sealing around different sizes of pipe. U.S. Pat. Nos. 3,572,627, 3,897,039, 3,897,040, 3,897,071 and 4,007,904 disclose annular blowout preventers. However, all annular blowout preventers have the disadvantage of requiring a large amount of strain energy to be stored in the packer element prior to applying the energy required to effect sealing pressure with the pipe or object and this is particularly evident when sealing about the smaller sizes of a pipe or object.
In spite of the many attempts to provide ram-type blowout preventers capable of effectively sealing around a range of pipe sizes against the relatively high pressures commonly encountered during drilling operations, there is no ram-type preventer presently available to fully satisfy this need.